"x == None" vs "x is None"

[Ulli Horlacher]

I have seen at several places "x == None" and "x is None" within
if-statements.
What is the difference? 
Which term should I prefer and why?


-- 
Ullrich Horlacher              Server und Virtualisierung
Rechenzentrum IZUS/TIK         E-Mail: horlacher at tik.uni-stuttgart.de
Universitaet Stuttgart         Tel:    ++49-711-68565868
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